A polymer emulsion is the suspension of polymers in a liquid. Typically, the polymers are in the form of micron-sized particles suspended in the liquid.
Coatings and films are commonly formed from polymer emulsions. As the liquid evaporates in the polymer emulsion, the suspended polymer solids come closer together until they touch and combine to form larger chains and eventually a film. Polymer emulsions are well known and widely used in the paint industry.
Polymer emulsions have also been used in the field of implantable and injectable medical technologies. In some cases, polymer emulsions have been used for coating medical devices. It has become appreciated that coatings can improve function of an implanted device at the site of implantation. For example, coatings for implantable medical devices can improve biocompatibility of the device surface, or provide a drug-delivery mechanism. U.S. Pat. No. 6,919,100 describes a process of applying an aqueous latex polymeric emulsion of vinylidenefluoride, hexafluoropropolyene, and an antiproliferative drug, and then drying the aqueous latex polymeric emulsion on the medical device to form a drug-releasing coating.
Polymer emulsions have also been used for the preparation of drug-containing microparticles. Microparticle formation using an emulsion-based process typically begins with the preparation of two separate liquid phases, first and second liquid phases. The first liquid phase is a solution (or dispersion) a drug in a solution of polymer dissolved in a first solvent. The second phase is a solution of surfactant and a second solvent. The second solvent is at least partially immiscible with the first solvent of the first phase. To form the emulsion, the first phase is combined with an excess amount of the second phase, generally using agitation or mixing. Microdroplets of the first phase become dispersed in the second (continuous) phase. The surfactant in the second phase can help to form and stabilize the dispersion of microdroplets in the second phase. The emulsion is then treated so that the polymeric material in the microdroplets hardens to form drug-containing polymeric microparticles. Often, to form the microparticles, the first solvent (which was originally used to dissolve the polymer) is removed by evaporation or extraction.
Unfortunately, emulsions are inherently unstable and undergo many destabilization phenomena, such as coalescence (particles combine to form a layer of liquid), flocculation (particle clumping), and creaming (particles concentrate near the top of bottom of the emulsion). Some of these can lead to phase separation of the materials in the emulsion. Destabilization of emulsions can be due to various causes, such as the type of polymer used, the type or amount of surfactant, and interaction of solid components in the emulsion. While surfactants can be added to stabilize emulsions, some may not be particularly effective or may not stabilize the emulsion for a desired period of time. Also, if the emulsion is used for formation of implantable or injectable material, some surfactants may not be suitable due to insufficient biocompatibility. In turn, destabilization can affect formation of coatings and microparticles. Loss of stability may also affect bioactive agent if included in the microparticles.